Radio frequency (RF) switches are placed in extreme conditions relative to switches operating in other technical regimes. In certain applications, these switches need to operate in the hundreds of gigahertz (GHz) range and handle upwards of one watt of input power in a linear fashion. In addition, RF switches need to exhibit a low on-state resistance to minimize power consumption and to avoid degrading the signals on which they operate. To achieve this level of performance, RF switches often exhibit extremely large widths relative to devices in alternative regimes. For comparison, current digital logic transistors have widths on the order of nanometers (nm) while the width of an RF switch transistor can be on the order of millimeters—differing by a factor of more than a million.
Due to the strenuous conditions in which they are required to operate, RF switches face more extreme versions of common problems faced by standard transistors, and are additionally burdened by a host of problems that do not appear in other technical regimes. For example, the accumulation of charge in the body of a metal-oxide-semiconductor (MOS) RF switch transistor can activate the MOS transistor's parasitic bipolar transistor. In this situation, the accumulated charge in the body of the device serves to bias the body of the MOS transistor as if it were the base of a bipolar transistor. This physical condition results in a performance degradation effect known as the “kink” effect. It is particularly problematic in RF switches implemented on semiconductor-on-insulator (SOI) wafers in which there is no external bias applied to the body of the transistor because there are no low impedance paths for the accumulated charge to follow in order to exit the body.
In addition to experiencing more problematic versions of common problems found in standard transistor technologies, RF switches face additional performance degradation from non-ideal physical conditions that are not problematic in other regimes. Accumulated charge is again also an example of this kind of physical condition. Given the large width of a standard RF switch, accumulated charge can introduce a nonlinear distortion to the signals on which the RF switch is operating while the RF switch is in the off state. While this may be negligible for transistors with small widths, the parasitic capacitance caused by accumulated charge aligned at the body junctions along the entire width of an RF switch has a significant negative effect on signals coupled to the terminals of the RF switch when it is in its off state.
RF switch transistors present a particular design challenge as compared to more standard transistors. Physical effects that cause performance degradation in standard transistors are felt more acutely in the RF regime. In addition, certain physical effects manifest themselves in performance degradation modes that are inconsequential outside of the RF regime. Therefore, device engineers working on RF switches employ specialized design methodologies and device configurations to deal with physical effects that are not generally a concern in other operating regimes.